Environmental protection and limited water availability have necessitated the adoption of larger temperature rises in the condensers of utility power plants. There has been increased use of cooling ponds and wet cooling towers (both natural and mechanical draft) and in some instances, dry cooling. An increase in turbine exhaust pressure has accompanied the adoption of these supplementary cooling systems. This not only reduces the plant efficiency but also places additional demands on the cooling system.
In the United States, dry cooling has been limited to one utility with an initial application on a 20 MW turbine and a subsequent 360 MW unit. Both of the applications were with air cooled condensers. South Africa has built six 665 MW units with air cooled condensers, with three more under construction. In other applications, indirect air cooling is used in which exhaust steam is channeled through a dry tower, usually a natural draft design. South Africa has built six 668 MW units using indirect air cooling. A number of smaller size indirect designs were built in England, Germany, Hungary, U.S.S.R., Iran, Brazil, Turkey, and South Africa. These plants employed either spray or surface condensers.
In at least one dry cooling study of a nuclear power plant, it was established that the use of multipressure or zoned condensers improved the plant economics. Moreover, the use of different size last row blades in each low pressure (LP) element (tandem compound six flow exhaust) further improved the economics. In this instance, the lowest pressure LP element had the largest exhaust annular area with decreasing annulus areas in the higher pressure LP elements. The economic benefit and improvement in turbine performance increases with the number of multipressure levels or zones. Under conventional practice the number of zones corresponded to the number of LP elements. However, U.S. Pat. No. 4,557,113 assigned to the assignee of the present invention, discloses a turbine system having separate zones in each half of a double flow LP element with downward exhaust. From the disclosed system, it is possible to obtain two zones with a single LP element, four zones with two double flow LP elements and six zones with three double flow LP elements.
U.S. patent application Ser. No. 07/317,495, filed Mar. 1, 1989, assigned to the assignee of the present invention, proposes to vary the gaugings on the last stage (rotating and stationary blades) by reorientating the blade foil while keeping the rotating blade profile the same to optimize the performance in the various zones of the LP turbines and to use different size last row blades in each half of a double flow LP element to achieve more optimum performance if the differences in exhaust pressure were large enough in the various zones. Turbines have been built in which the individual LP turbines of a specific unit have different length last row blades.
With dry finned tubes of air cooled condensers, the temperature of the cooling air rises substantially. The gradient for the transfer of heat is the difference in temperature of the air and the condensing steam. The tubes of the dry finned sections must be comparatively shallow, which means that usually not more than three to six rows of tubes are crossed in succession by the air passing over them. The successive increase in air temperature will produce a successively higher steam condenser pressure in each row, although this is sometimes ameliorated by varying the fin spacing of each row.
The different condensing pressures must equalize in the headers so that: (1) the condensate from all tubes will drain completely; and (2) the air in all tubes will be separated and evacuated. In one exemplary system, the air cooled condenser operates at approximately 15.degree. C. lower saturation temperature owing to pressure loss in the steam duct (connecting the turbine exhaust flange and the air cooled condenser) and the condensing elements.
Because of the tendency of the air cooled condenser to produce successively higher steam condenser pressures in each row of tubes (as the air successively increases in temperature in passing through the air cooled condenser), it is especially suited to multi-pressure or zoning operation. In this case, the lowest pressure zone would occur in the first row of tubes and the highest pressure zone in the last row of tubes.
In May, 1979 the assignee of the present invention was granted a patent on a zoned or multipressure system for a "Dry Cooling Plant System" (U.S. Pat. No. 4,156,349). In this instance, the LP steam turbines exhausted to steam condensers-ammonia reboilers. The ammonia evaporated, was ducted to the air cooling tower where it condensed, and returned to the condenser-reboiler. In this instance, the ammonia from one condenser-reboiler went to the cooling tower tubes that received the inlet cooling air. The ammonia from the other condenser-reboiler went to the cooling tower tubes that received hot air leaving the first group of tubes. So, the steam turbine operated with two pressure zones on a dry cooled plant.
It was noted that increasing the number of condensing zones or pressure levels improves cycle performance and economics of indirect air cooled plants because of the large cooling range (large temperature rise) typical of dry cooled systems. In the case of air cooled condensers, there is an inherent tendency for each row of condenser tubes to operate at successively higher pressure as the air passes through the condenser system.
Moreover, many wet cooling systems with conventional steam condensers have large temperature rises and are especially suited to multi-pressure or zoned condenser applications. As noted earlier, increasing the number of pressure zones improves performance on both indirect air cooled and wet cooling tower plants. The problem is that the number of zones is limited to the number of turbine exhaust flows. The aforementioned U.S. Pat. No. 4,557,113 discloses a system in which two zones are obtainable on a double flow LP element, i.e., a condenser is divided into two zones with exhaust from one end of the turbine coupled to one of the zones and exhaust from the other end of the turbine coupled to the other of the zones. The advantages of this two zone system suggest that more zones might provide additional improvement. However, it has been believed that the number of zones is limited to the number of available turbine exhausts.
If it were possible to obtain more than two exhaust pressure zones on a double flow LP element or multiple pressure zones on a single flow LP element, additional improvements could be obtained. Table I illustrates the pressure levels and increase in available energy from use of a low pressure zone in a two zone single flow LP element over single pressure operation, both designs having a 20.0.degree. C. temperature rise of the cooling water. T.sub.0 is the incoming cooling water temperature. T.sub.2 is the cooling water outlet from the second zone of a multi-pressure, two zone condenser. P.sub.2 and P.sub.1 are the saturation (condensing) pressures corresponding to T.sub.2 and T.sub.1, respectively. The portion of the exhaust steam (approximately half) that exhausts to the low pressure zone has between 15.5 and 16.4 Kcal/Kg more available energy than the steam in the single pressure design. The increase in available energy is dependent upon the initial condenser temperature which was varied between 30.degree. C. and 56.7.degree. C., corresponding to a range of water temperatures leaving a cooling tower.
In Table II, a single pressure and a four pressure zoned condenser are compared. In this case, T.sub.0 is the initial cooling water temperature with T.sub.4 being the water temperature leaving the last zone. T.sub.1, T.sub.2, and T.sub.3 are the water temperatures leaving the other zones. P.sub.1, P.sub.2, P.sub.3, and P.sub.4 are the condensing pressures in the various zones. P.sub.4 is also the condensing pressure of an unzoned or single pressure design. There are corresponding increases in available energy of the steam expanding in the various zones above the available energy of the single pressure design.
Tables III and IV relate to comparisons between one zone and two zone and one zone and four zone designs, respectively, for a temperature rise of 13.3.degree. C. The temperature rises in dry cooling systems would probably approach the 20.0.degree. C. level while the 13.3.degree. C. to 20.0.degree. C. range would be more typical of natural draft wet cooling towers. Fossil units with natural draft wet cooling towers would tend to be in the lower half of the 13.3.degree. C. to 20.0.degree. C. range while nuclear units would be in the upper half of this range. Fossil applications with wet type mechanical draft cooling towers generally have temperature rises between 8.3.degree. C. and 13.9.degree. C. while nuclear plants with mechanical draft towers would usually have temperature rises between 13.3.degree. C. and 16.7.degree. C. In areas with low humidity, mechanical draft wet towers have been built with temperature rises of 16.7.degree. C. to 20.0.degree. C.
Tables V and VI identify the steam temperatures and pressures in the various zones for single, two, and four zone combinations with 13.3.degree. C. and 20.0.degree. C. temperature rises and given conditions in the maximum pressure zone.
Calculations were made with the standard hood loss on the turbine configuration utilized to evaluate zoning as well as with 0.56, 1.11, and 1.67 Kcal/Kg hood loss increases. Table VII compares single or unzoned performance with two zone performance, and 13.3.degree. C. temperature rises. The two zone performance is presented with 0, 0.56, 1.11, and 1.67 Kcal/Kg increases in hood loss. Table VIII presents comparable data but with a 20.0.degree. C. temperature rise.
Both of these comparisons relate to a single flow LP section. Even with a 1.67 Kcal/Kg increase in hood loss, there is still an output increase with two zones. The increase in output is larger with a 20.0.degree. C. rise than with a 13.3.degree. C. rise.
If the turbine had a double flow LP element, it could be built with two zones as shown in the aforementioned U.S. Pat. No. 4,577,113. For that design, there would be no increase in hood loss for a given exhaust volumetric flow.
It is obvious that there is a significant increase in available energy with multi-pressure. For the case of two versus one zone, the increase is between 7.72 and 8.22 Kcal/Kg for a 20.0.degree. C. rise and 5.33 to 5.61 Kcal/Kg for a 13.3.degree. C. rise, based on the total exhaust flow (half of value shown on Tables I and III). In the case of four versus one zone, the increase is between 11.6 and 12.3 Kcal/Kg for a 20.0.degree. C. rise and between 8.06 and 8.39 Kcal/Kg for a 13.3.degree. C. rise, based on the total exhaust flow (half of value shown on Tables II and IV).
Tables V and VI identify the pressures associated with the various zoning configurations for various maximum condensing temperatures and condenser temperature rises of 13.3.degree. C. and 20.0.degree. C.